A) Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a tunable twin-guide distributed feedback (TTG-DFB) type laser.
B) Description of the Related Art
Present backbone transmission systems for large capacity optical communication networks utilize wavelength division multiplexing (WDM) which can increase the capacity of transmission by multiplexing optical signals on a wavelength axis. As the degree of multiplexing is made high, the number of semiconductor lasers to be used as light sources increases. Backup light sources are also required as many as or larger than that of semiconductor lasers, resulting in annoying stock management. Simplifying the management of tunable optical light sources has been desired. Wavelength routing techniques have been paid attention, which dynamically change a routing destination depending upon the wavelength.
A distributed Bragg reflector (DBR) laser is known as one example of tunable lasers. FIG. 4A is a cross sectional view of a conventional tunable DBR laser. On a lower clad layer 100, an active layer 101 and an upper clad layer 103 are formed. Along the longitudinal direction of an optical resonator, a gain region G, a phase adjusting region P and a DBR region R are defined in this order. In the DBR region R, a diffraction grating 102 is disposed near the active layer 101.
Current is supplied to the gain region G by an electrode 104, to the phase adjusting region P by an electrode 105, and to the DBR region R by an electrode 106. As the current is supplied to the gain region G, laser oscillation occurs. As the current is injected into the DBR region R, the oscillation wavelength can be changed. The oscillation wavelength is determined by the longitudinal mode nearest to the wavelength at which the loss of an optical resonator is minimum, so that the wavelength change is discrete. By adjusting the injection amount of current into the phase adjusting region P, it is possible to realize a quasi-continuous wavelength change.
With this method, however, it is necessary to control both the injection currents into the DBR region R and phase adjusting region P, resulting in a complicated control system. When the relation between wavelength and current shifts after a long time operation, it is difficult to follow a change in wavelength, leaving a reliability problem.
A TTG- DFB laser is known as a semiconductor laser which can eliminate the difficulty in controlling a tunable DBR laser. Refer to “Tunable twin-guide laser: A novel laser diode with improved tuning performance” by M. C. Amann, S. Illek, C. Schanen, and W. Thulk, Applied Physics Letters, Vol. 54, (1989), pp. 2532–2533.
FIG. 4B is a cross sectional view of a TTG-DFB laser. A tuning layer 111, an intermediate layer 112, an active layer 113, a diffraction grating 114 and an upper clad layer 115 are stacked in this order on a substrate 110 also serving as a lower clad layer. From the viewpoint of optical transmission mode, the layers of the tuning layer 111 to the diffraction grating 114 are disposed near each other so that they exist in the same mode. An electrode 117 is formed on the bottom surface of the substrate 110, and an electrode 116 is formed on the upper clad layer 115.
The intermediate layer 112 has a conductivity type opposite to that of the substrate 110 and upper clad layer 115. The tuning layer 111 and active layer 113 are electrically independent from each other. Namely, current injected from the electrode 117 into the tuning layer 111 is controlled independently from that injected from the electrode 116 into the active layer 113.
As current is flowed in the active region 113, laser oscillation can be excited. As current is flowed in the tuning layer 111, a refractive index of the tuning layer 111 changes due to the plasma effect. Because the effective refractive index in the laser transmission mode also changes, the Bragg wavelength determined by the diffraction grating 114 changes so that the oscillation frequency changes. In this manner, the oscillation wavelength can be changed in a continuous manner by controlling only the current to be injected into the tuning layer 111.
FIG. 4C is a cross sectional view showing another structure of a TTG-DFB laser. In the laser shown in FIG. 4B, the diffraction grating 114 is disposed on the active layer 113 on the side opposite to the intermediate layer 112. In the laser shown in FIG. 4C, a diffraction grating 114 is disposed on a tuning layer 111 on the side opposite to an intermediate layer 112.